The present invention generally relates to a photodetector and, more particularly, to the photodetector suited for use as an optical pick-up head for optically recording or reproducing information on or from an optical information carrier medium.
An optical video disc system is nowadays well-known wherein information recorded on an optical information carrier medium, for example, a video disc, is optoelectrically reproduced. In general, the optical video disc system makes use of an optical pick-up head and a laser as a source of light to be eventually detected by the pick-up head.
FIG. 7 of the accompanying drawings illustrates a popular example of optical pick-up head system, wherein reference numerals 1, 2, 3, 4, 5, 6, 7 and 8 represent a semiconductor laser, a diffraction grating, a beam splitting prism, an objective lens assembly, a concave lens, a cylindrical lens, a photodetector, and a video disc, respectively. A tracking control system used therein is a three-beam tracking control system, wherein a tracking error signal can be detected by a pair of photocells 7a and 7b for tracking purpose formed in the photodetector 7. On the other hand, a focus control system is an astigmation system, wherein a focusing error signal can be detected by a four-segment photocell 7c. An information (RF) signal can be detected by the four-segment photocell 7c.
In this type of optical pick-up head, the photodetector 7 is of a type formed by molding a synthetic resin, for example, transparent epoxy resin, the structure of which is shown in FIGS. 8 and 9. As shown therein, the photodetector 7 comprises a substrate 7d on which the photocells 7a, 7b and 7c are formed, lead wires 7e feeding respective outputs from the photocells, and output terminal members 7f for connecting the photocells with external circuit wirings, all of said substrate 7d, the lead wires 7e and portions of the terminal members 7f being embedded in a molded transparent resin layer 7g. Built-in functional units are arranged at a substantially central portion of the molded resin with respective light receiving faces 7a', 7b' and 7c' of the photocells held parallel to a light receiving surface 7h of the resin layer 7g.
Outputs (currents) from the associated photocells 7a and 7b for tracking purpose are converted into respective voltages by a tracking signal processing circuitry shown in FIG. 10. The tracking error signal is obtained from a subtracting circuit A2 by subtracting the respective outputs of the photocells 7a and 7b. More specifically, assuming that the outputs (outputs after having been amplified) of the photocells 7a and 7b are expressed by Sa and Sb, respectively, a tracking error signal component TE can be expressed by TE=Sb-Sa. It is to be noted that, in FIG. 10, reference characters A0 and A1 represent amplifiers, FIG. 11(a) illustrates the respective waveforms of the outputs Sa and Sb, whereas FIG. 11(b) illustrates the waveform of the tracking error signal. Normally, an intermediate point (1/2 of the value P-P) of the TE signal does not coincide with the GND level because of conditions of an optical system as well as the difference in DC level of the outputs Sa and Sb and also in output level, and for compensating for a deviation of the intermediate point by means of a circuit, either the resistance R4 or R1 or the resistance RG or R3 is adjusted to bring the intermediate point into coincidence with the GND level.
However, since a tracking servo is apt to function to bring the intermediate point into coincidence with the GND level in the event of deviation occurring between the intermediate point and the GND level, it will have an offset in a tracking direction when the deviation exceeds a predetermined quantity, and in this case the servo will no longer function.
The above described conventional optical pick-up head has a problem in that, since it often happens that the tracking error signal changes with change in temperature to such an extent that a serve circuit can no longer function its control, the tracking operation tends to become unstable. FIG. 12 illustrates a temperature-dependent change of the tracking error signal, and as shown therein, the tracking error signal TE once adjusted at normal temperature undergoes a change with change in temperature T and, at a certain temperature, displaces to a position shown by TE'. In this connection, assuming that the amplitude of the tracking error signal is expressed by P and the displacement of an intermediate point C of the amplitude of the tracking error signal is expressed by .DELTA.x, and when the rate of change .alpha.(=(.DELTA.x/P).times.100%) attains a value higher than 10%, the tracking serve tends to become unstable.
FIG. 13 illustrates the temperature dependent characteristic of the rate of change .alpha. in the conventional optical pick-up head, and it has been found that the change rate .alpha. often attains 15 to 20% at maximum and, moreover, the change rate .alpha. varies in swelling fashion at a predetermined interval of temperature, for example, by the effect of a temperature difference of 10.degree. to 15.degree. C. In FIG. 10, where the initial adjustment has been done at normal temperature in which case the change rate .alpha. is, as a matter of course, zero (because the intermediate point coincides with the GND level and the value .DELTA.x is therefore zero), the change rate .alpha. attains a peak value at a temperature 10.degree. to 15.degree. C. higher than the normal temperature. Accordingly, at a temperature as high as the temperature at which the change rate .alpha. exceeds 10%, the tracking operation tends to become unstable.
It has been found that the above described problem has resulted from the interference of light which is attributable to the change in optical path induced by the change in temperature. The interference of light is paramount particularly in the photodetector, occupying 70 to 80% of all the change rate.
FIG. 14 is a diagram used to explain how the light interference occurs as a result of difference in optical path, and a beam B0 of light for tracking purpose enters the light receiving surface 7h of the molded resin layer and is subsequently detected by the photocell 7a. Assuming that the position of the light receiving surface of the molded resin layer is located at a level 7h at the time the position and the intensity of the incident beam have been initially adjusted at normal temperature, and when the temperature has increased to a value higher than the normal temperature by a predetermined value, the molded resin layer undergoes a thermal expansion with the light receiving surface consequently shifted to a position shown by 7h'.
Let it be assumed that the distance between the light receiving face of the photocell 7a and the light receiving surface 7h is expressed by l1, and the distance between the light receiving surfaces 7h and 7h' (attributable to the thermal expansion of the molded resin layer) is expressed by .DELTA.l. The incident beam B0 after having impinged upon the light receiving face of the photocell 7a is partially reflected therefrom towards the light receiving surface of the molded resin layer as shown by B2 or B2', and when and after the distance .DELTA.l has attained a value which brings about the interference of light, the reflected light interferes with the incoming incident beam B0 producing a pattern of dark and bright fringes on the light receiving face of the photocell 7a.
It is well known from the theory of interference of light that, when two light beams from the same light source and, hence, of the same wavelength .lambda. travel along respective optical paths of different distance having a path difference of half the wavelength .lambda., a pattern of alternate dark and bright bands or fringes is produced.
Accordingly, the interference occurs when the relationship expressed by the following equation is satisfied: EQU 2(l1+.DELTA.l)=n.multidot..lambda./2 (1)
wherein n is a positive integer other than 0. Referring to the photocell 7a shown in FIG. 14, when n=2m and n=2m+1 (wherein m is a positive integer), the interference of bright fringes and that of dark fringes occurs, respectively. In other words, in the case where the amount of change .DELTA.l in thickness continuously varies with change in temperature, the interference of the dark and bright fringes alternating at intervals of a quarter wavelength is produced with the consequence that the photocell 7a generates a DC current of varying level.
The description similar to that set forth above in connection with the photocell 7a can be equally applicable to the photocell 7b particularly when the relationship expressed by the following equation is satisfied: EQU 2(l2+.DELTA.l)=n.multidot..lambda./2 (1)
However, because of the resin molded product the distances l1 and l2 do not become equal to each other in view of the fact that each value of the distances l1 and l2 is in the order of submicorn, and, therefore, it often occurs that the phase of the interference occurring in one of the photocells 7a and 7b does not match with that of the other of the photocells 7a and 7b. More specifically, assuming for the sake of brevity that no interference of light occurs in the photocell 7b, the following relationship will be established: EQU .DELTA.TE.sub.DC .apprxeq.k.multidot.(7a).sub.DC k: constant
and, as shown in FIG. 13, the rate of change .alpha. attributable to the varying level of the DC current produced from the photocell 7a will vary. Assuming that, in FIG. 13, the bright tringes and the dark fringes are produced at respective temperatures T1 and T2, the change in temperature from the value T1 to the value T2 results in the change in the value .DELTA.l in a quantity corresponding to the fourth of the wavelength .lambda. as can be understood from equation (1). The graph of FIG. 13 applies where the thickness of the resin molded layer having a coefficient of thermal expansion which is 5.2.times.10.sup.-5 .degree. C. is 600 .mu.m and the wavelength .lambda. of the laser beam is 800 nm. In view of the fact that the value .DELTA.l per increase of 1.degree. C. is: .DELTA.l=600.times.5.2.times.10.sup.-5 .apprxeq.31 nm, and will be equal to one fourth of the wavelength .lambda. when the temperature increases about 6.degree. C., it coincides with the result of experiments which show that the half cycle (T1.fwdarw.T2) corresponds to the temperature increase of 5.degree. to 7.5.degree. C.
Where the distances l1 and l2 above the respective photocells 7a and 7b embedded in the resin molded layer has the following relationship: EQU l1.about.l2=(2m+1).multidot..lambda.4
the phases of interference are displaced 180.degree. and, accordingly, the change rate .alpha. or TE.sub.DC attains a maximum value.
In an attempt to substantially obviate the above discussed problems inherent in the conventional device, Japanese Laid-open Utility Model Publication No. 56-157762, laid open to public inspection 1981, has proposed to use a light reflecting film on the surface of the resin molded product. However, it has been found problematic in that a complete bond between the film and the resin surface cannot be achieved without difficulty and, also, the heat treatment used to form the film often results in the deterioration (for example, reduction in light transmissivity) of the resin molded body.